Shortened process for imparting corrosion resistance to aluminum substrates

ABSTRACT

A process for importing corrosion resistance to the surface for an aluminum substrate by contacting the aluminum substrate with an aqueous solution containing at least one of terafluoroboric acid, hexafluorosilisic acid, hexafluorotitanic acid, hexafluorozirconic acid, hexafluorohafnic acid and soluble, partially or totally neutralized salts and vanadate anions.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from U.S. provisional application Serial No. 60/237,305 filed Oct. 2, 2000, the disclosure of which is incorporated herein by reference.

FIELD AND BACKGROUND OF THE INVENTION

[0002] Many processes for imparting excellent corrosion resistance to an aluminum substrate, which term unless explicitly limited below is intended to include all alloys that include at least 55% by weight of aluminum, are known. All of them previously known to give high quality results, whether the treated metal is to be used without further processing or is to be further coated with a typical organic protective coating such as paint, require multiple process operations. These operations often include pickling and/or deoxidizing, acid or alkaline cleaning, formation of a conversion coating, and a post treatment of the conversion coating formed. Furthermore, almost all of these primary operations must normally be followed by at least one water rinse, and these rinsing stages require their own process equipment and time that further contribute to the overall cost of a metal treatment process that uses them.

[0003] There has accordingly been a need felt in the art for shorter process sequences that could give acceptable corrosion protection where the well established process sequences that produce the best results currently commercially available are considered too expensive. Heretofore, however, there has been little success reported in meeting this need.

[0004] A major object of this invention is to provide such a shortened but still high quality corrosion protection process and suitable treatment liquids for use in the process. A more particular object is to provide such a process for the treatment of aluminum substrates that, because of their intended use, can not practically be protected further by painting after a coating according to this invention is formed on the substrates. The most important commercial use of this type of aluminum is in heat exchangers, for which an organic coating such as paint would limit the heat conductivity too much.

[0005] Except in the claims and the specific examples, or where otherwise expressly indicated, all numbers in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred, however. Also, throughout this specification, unless expressly stated to the contrary: percent, “parts of”, and ratio values are by weight; the term “polymer” includes “oligomer”, “copolymer”, “terpolymer”, and the like; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description, or as reduced or increased in amount in situ by chemical reactions explicitly stated in the description, and does not necessarily preclude unstated chemical interactions among the constituents of a mixture once mixed; specification of materials in ionic form additionally implies the presence of sufficient counterions to produce electrical neutrality for the composition as a whole (any counterions thus implicitly specified should preferably be selected from among other constituents explicitly specified in ionic form, to the extent possible; otherwise such counterions may be freely selected, except for avoiding counterions that act adversely to any of the objects of the invention); and the term “mole” means “gram mole” and the term itself and its grammatical variations may be applied to elemental, ionic, unstable, hypothetical, and any other chemical species defined by number and type of atoms present, as well as to compounds with well defined molecules.

BRIEF SUMMARY OF THE INVENTION

[0006] It has been found that commercially acceptable corrosion performance as measured by accelerated corrosion testing can be achieved on aluminum substrates by a process with no more than three process operations, when the final process operation is treatment with an aqueous liquid treatment solution that contains both vanadate anions and complex fluorides selected from the group consisting of tetrafluoroboric acid, hexafluorosilicic acid, hexafluorotitanic acid, hexafluorozirconic acid, hexafluorohafnic acid, and the water soluble partially or totally neutralized salts of all of the acids recited earlier in this sentence.

DETAILED DESCRIPTION OF THE INVENTION

[0007] A process according to the invention includes at a minimum an operation of bringing an aluminum substrate to be treated into contact with an aqueous liquid treatment solution that comprises, preferably consists essentially of, or more preferably consists of, in addition to water:

[0008] (A) a concentration of a total of dissolved tetrafluoroboric acid, hexafluorosilicic acid, hexafluorotitanic acid, hexafluorozirconic acid, hexafluorohafnic acid, and the water soluble partially or totally neutralized salts of all of tetrafluoroboric, hexafluorosilicic, hexafluorotitanic, hexafluorozirconic, and hexafluorohafnic acids, the concentrations of any salts included in the total being measured as their stoichiometric equivalent as the corresponding acid, that preferably is at least, with increasing preference in the order given, 0.10, 0.30, 0.50, 0.55, 0.60, 0.65, 0.70, 0.75, 0.80, 0.85, 0.90, or 0.95 millimoles per kilogram of an aqueous liquid treatment solution (this unit of concentration being freely used hereinafter for any constituent of any continuous homogeneous phase and being hereinafter usually abbreviated as “mM/kg”) and independently preferably is not more than, with increasing preference in the order given, 20, 17, 14, 11, 9, 7, 5, 3.0, 2.5, 2.0, 1.7, 1.4, 1.2, or 1.0 mM/kg; and

[0009] (B) a concentration of vanadate anions, measured as its stoichiometric equivalent as vanadium atoms, that is at least, with increasing preference in the order given, 0.40, 0.80, 1.5, 2.0, 2.3, 2.6, 2.9, 3.2, 3.5, 3.8, 4.0, 4.2, 4.4, or 4.6 mM/kg and independently preferably is not more than, with increasing preference in the order given, 95, 75, 55, 45, 35, 25, 20, 15, 13, 11, 9, 8.0, 7.0, 6.5, 6.0, 5.5, or 5.0 mM/kg.

[0010] The source(s) of component (A) as described above preferably are selected from the group consisting of hexafluorotitanic, hexafluorozirconic, and hexafluorohafnic acids and their salts, and still more preferably are selected from the group consisting of hexafluorozirconic and hexafluorohafnic acids and their salts (with hexafluorohafnic acid and its salts normally being present only as an impurity in the hexafluorozirconic acid and its salts). In order to supply acidity that is useful to the process, the acids are preferably used rather than their salts.

[0011] The source(s) of component (B) as described above preferably are water soluble salts that include any of the vanadate and condensed vanadate ions and more preferably are water soluble salts that include decavanadate ions with the chemical formula V₁₀O₂₈ ⁻⁶. When such salts are dissolved, they are believed to give rise to some protonated derivatives having the general formula V₁₀O_((28−i))(OH)_(i) ^(−(6−i)), where i represents an integer from one to four, which are believed to be the predominant species present in aqueous solutions with a pH from 2 to 6. Cf. F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 4th Ed., (John Wiley & Sons, New York, 1980), p. 712. Irrespective of any such equilibria with protonated anions or even anions with lower degrees of condensation that may result from reaction in solution, component (B) is most preferably sourced to the composition from decavanadate salts only. Primarily for economic reasons, the specific salt sodium ammonium decavanadate is preferred, because it is the cheapest commercially available decavanadate salt.

[0012] An aqueous liquid treatment solution according to the invention preferably has a pH value that is at least, with increasing preference in the order given, 0.5, 1.0, 2.0, 2.5, 3.0, 3.3, 3.5, or 3.7 and independently preferably is not more than, with increasing preference in the order given, 5.5, 5.2, 5.0, 4.8, 4.6, 4.4, or 4.2.

[0013] During its contact with an aluminum substrate being treated, the above-described aqueous liquid treatment solution according to the invention preferably is maintained at a temperature that is at least, with increasing preference in the order given, 30, 35, 40, 45, 50, or 52° C. and independently preferably is not more than 95, 90, 85, 80, 75, 70, 68, or 66° C. Additionally, the time of contact between the aluminum substrate and the above described aqueous liquid treatment solution according to the invention preferably is at least, with increasing preference in the order given, 0.5, 1.0, 2.0, 3.0, 4.0, 4.5, or 5.0 minutes (hereinafter usually abbreviated as “min”) and independently preferably is not more than, with increasing preference in the order given, 30, 25, 20, 15, 13, 11, 9.0, 8.0, or 7.5 min. Independently also: the “coating weight” of zirconium, i.e., the milligrams of zirconium included in the coating per square meter of the substrate surface treated, this unit being hereinafter usually abbreviated as “mg/m²” and being applicable also to any other coated material, preferably is at least, with increasing preference in the order given, 10, 20, 30, 35, 40, 45, 50, 55, or 60 mg/m² and independently, primarily for economy, preferably is not more than, with increasing preference in the order given, 500, 300, 200, 150, 145, 140, or 135 mg/m²; and the coating weight of vanadium preferably is at least, with increasing preference in the order given, 0.10, 0.20, 0.30, 0.40, or 0.50 and independently, primarily for economy, preferably is not more than, with increasing preference in the order given, 10, 8, 6, 4, 2.0, 1.5, or 1.1 mg/m².

[0014] The above described necessary process operation according to the invention may be preceded by a deoxidizing operation followed by a water rinse operation. These preliminary operations are preferred if the aluminum substrate to be coated has visible heavy scale and/or organic soil. However, if the aluminum substrate surface to be treated appears reasonably clean visually, as is most often true in treating aluminum surfaces to be used as heat exchangers in vehicle radiators, air conditioners, and the like, it is preferable not to deoxidize or otherwise vigorously clean the substrate before subjecting it to the necessary process operation according to the invention as described above, because the corrosion protection obtained is actually better in the absence of vigorous pre-cleaning. In such circumstances, a genuinely single operation treatment process is possible.

[0015] After contact with an aqueous liquid treatment solution according to the invention as described above, the treated substrate surface may be rinsed with water and if so rinsed is preferably rinsed with deionized or similarly purified water. However, as with pre-cleaning, ordinarily post-rinsing degrades the corrosion resistance otherwise obtainable.

[0016] A preferred aqueous liquid treatment solution according to the invention is quite dilute. Therefore, to avoid the cost of shipping large amounts of water that can usually be supplied more cheaply at the point of use, a preferred aqueous liquid treatment solution according to the invention will usually be made in practice by diluting a concentrate composition that contains all of the ingredients other than water in concentrations that preferably are at least, with increasing preference in the order given, 2, 4, 6, 8, 10, 12, or 14 times the concentrations given above for a working solution as actually used in a process according to the invention. Independently, primarily in order to avoid possible instability of the solution, a concentrate composition preferably contains all of the ingredients other than water in concentrations that are not more than, with increasing preference in the order given, 200, 150, 100, 50, 25, or 20 times the concentrations given above for a working solution as actually used in a process according to the invention. Concentrate compositions of this type are also within the scope of this invention.

[0017] The invention and its benefits may be further appreciated from consideration of the following, non-limiting, examples and comparison examples.

Group 1, with Accelerated Corrosion Testing Results

[0018] A concentrate composition according to the invention was made by mixing 10 parts of sodium ammonium decavanadate and 20 parts of a 20% solution in water of hexafluorozirconic acid with 970 parts of deionized or similarly purified water. A preferred working aqueous liquid treatment solution according to the invention was made by mixing this preferred concentrate composition with 19 times its own mass of deionized or similarly purified water and adjusted to a pH of 4.0 with aqueous ammonia. This preferred aqueous liquid treatment solution was used in the Process Examples described in Table 1 below. The substrates in these and the Comparison Process Examples were sections of commercial vehicle radiator core stock made from one or more of the aluminum alloys 4004, 4104, 4343, 4045, 4047, 3003, 3005, 1050, or 3102. The sheets of aluminum were reported by their supplier to be joined together by either vacuum brazing for types 4004, 4104, and 3003 or flux brazing for the other noted aluminum alloys to make the radiator core stock. Both vacuum and flux brazing are standard techniques in the art.

[0019] Process examples according to the invention and comparison examples are described in Table 1 below. After completion of the process and drying of the treated substrates, the treated substrates were tested by salt spray accelerated corrosion testing TABLE 1 Process Operation Process or Test Details for: Number in Comparison Example Chronological (Prior Art Example Number: Order (Prior Art Standard) 1.1 1.2 1.3 1 Nitric and Deoxidize with Contact for 5.0 min at Contact for 6.0 min at hydrofluoric acid DEOXALUME ® 60° C. with preferred 60° C. with preferred cleaner for 1.0 min at technology for 2.0 working composition working composition 38° C. min at 38° C. described last above described last above 2 Water rinse at Water rinse at Rinse with deionized None ambient temperature ambient temperature water at ambient for 30 seconds for 30 seconds temperature for 30 seconds 3 Chromium-free Contact for 6.0 min None None conversion coating at 60° C. with (using ALODINE ® preferred working 5200 technology) for composition 2.0 min at 49° C. described last above 4 Water rinse at None None None ambient temperature for 30 seconds 5 Treat with solution of None None None sodium ammonium decavanadate in water for 2.0 min at 60° C. 6 Rinse with deionized None None None water at ambient temperature for 30 seconds Results After Salt Spray for Vacuum Brazed|Flux Brazed Substrates after:  168 hours 5.5|6.0 n.m. 4.5|5.0 n.m.  336 hours 5.0|4.0 n.m. 4.0|4.0 n.m. 1000 hours 3.5|4.5 4.0|5.0 4.0|4.0 4.0|5.0

[0020] according to American Society for Testing and Materials Procedure B-117. Some results of this testing are also reported in Table 1 below, in which “n.m.” means “not measured”. The results are reported with a number according to the following scale:

[0021] 7=same appearance as before testing;“

[0022] 6=no spotting, but some readily visible change from appearance before testing;

[0023] 5=lightly spotted, but unspotted surface still shiny;

[0024] 4=moderately spotted, remaining surface only dimly shiny;

[0025] 3=heavily spotted, no shine;

[0026] 1 or 2=very poor appearance, with 1 somewhat worse than 2.

Example Group 2, With Temperature and Time of Contact Variations

[0027] In this group, the working composition was the same as in Group 1, but the substrates were rectangular test plates of Type 3003 aluminum alloy. Some of the substrates were cleaned and rinsed as for Example 1.1 of Group 1; the others were not cleaned at all. Contact between the test substrates and the working aqueous liquid treatment composition was for the times and at the temperatures shown in Table 2 below. The treated substrates were not rinsed but were dried after this treatment.

[0028] After drying, the amounts of zirconium and vanadium on the treated surfaces were determined by X-ray induced emission spectroscopy with a PORTASPEC™ Model 2501 apparatus available from Cianflone Scientific Company, Cannonsburg, Pa., USA. In this apparatus, there is an X-ray tube which emits a beam of primary radiation onto the sample to be analyzed. The primary radiation causes the atoms in at least the surface region of the sample to emit secondary fluorescent radiation which contains lines characteristic for each element present in the emitting region. This secondary radiation is directed through a collimator onto a large single crystal within the apparatus. The single crystal acts as a diffraction grating to separate the various wavelengths present in the secondary radiation. The entire angular range of the diffracted secondary radiation emitted from the sample is scanned by a detector in the apparatus and may be read as “counts” on a meter that is also part of the apparatus. The intensity of the radiation at the wavelengths characteristic of zirconium or vanadium is, with suitable corrections, proportional to the number of zirconium or vanadium nuclei within the emitting region of the sample. The number of counts can be converted to mg/m² of the counted metal nuclei, after standardization with samples containing known amounts of the two metals on their surfaces. A blank value for the untreated substrate was determined for each metal and subtracted from the readings obtained for the treated substrates. The resulting values are also shown in Table 2. The amount of coating formed was not very sensitive to time and temperature within the range shown, except possibly for the lowest time and temperature shown in Table 2, for which the coating weights (except for vanadium on a cleaned surface) were notably lower than for the other conditions shown in Table 2. TABLE 2 Process Characteristics Treatment Coating Weight in mg/m² of: Solution Contact Zirconium Vanadium Temperature, Time, Not Not ° C. Minutes Precleaned Precleaned Precleaned Precleaned 38 2.0 50 14 0.50 0.11 5.0 89 60 0.51 0.46 8.0 98 63 0.47 0.51 52 2.0 84 40 0.55 035 5.0 110 86 0.62 0.76 8.0 103 82 0.74 0.61 66 2.0 84 61 0.80 0.80 5.0 115 108 1.00 0.90 8.0 135 79 0.83 0.86 

The invention claimed is:
 1. A process for imparting corrosion resistance to a surface of an aluminum substrate, which comprises contacting the surface of the aluminum substrate with liquid treatment solution which comprises: water, and (A) 0.1 to 20 mM/Kg of a fluorine containing composition selected from the group consisting of tetrafluoroboric acid, water soluble, partially or totally neutralized salts of tetrafluoroboric acid, hexafluorotitanic acid, water soluble, partially or totally neutralized salts of hexafluorotitanic acid, hexafluorozirconic acid, water soluble, partially or totally neutralized salts of hexafluorozirconic acid, hexafluorohafinic acid, water soluble, partially or totally neutralized salts of hexafluorohafinic acid and mixtures thereof; the concentrations of any salts included in the solution being measured as their stoichiometric equivalent of the corresponding acid; and (B) from 0.40 to 95 mM/kg. of vanadate anions, measured as its stoichiometric equivalent as vanadium atoms.
 2. The process of claim 1 wherein the liquid treatment solution contains from 0.55 to 11 mM/kg of the fluorine containing composition.
 3. The process according to claim 1, wherein the solution contains from 1.5 to 55 mM/Kg of vanadate anions.
 4. The process of claim 1 wherein the solution contains from 0.55 to 11 mM/Kg of the fluorine containing composition and from 1.5 to 55 mM/Kg of vanadate anions.
 5. The process of claim 1 wherein the liquid treatment solution is at a pH of from 0.5 to 5.5.
 6. The process of claim 5 wherein the pH is from 2.5 to 4.8.
 7. The process of claim 1 wherein a temperature of the liquid treatment solution is from 30° C. to 95° C.
 8. The process of claim 7 wherein the temperature of the treatment solution is from 40° C. to 80° C.
 9. The process of claim 4 wherein a pH of the liquid treatment solution is from 1.0 to 5.0.
 10. The process of claim 9 wherein the pH of the treatment solution is from 2.0 to 4.6.
 11. The process of claim 4 wherein the liquid treatment solution is at a temperature of from 30° C. to 95° C.
 12. The process of claim 5 wherein a temperature of the liquid treatment solution is from 30° C. to 80° C.
 13. The liquid treating solution useful in the process of claim
 1. 14. The liquid treating solution of claim 13 comprising 0.55 to 11 mM/Kg of the fluorine containing composition and 1.5 to 55 mM/Kg of vanadate anions.
 15. The liquid treating solution of claim 14 having a pH of from 0.5 to 5.5.
 16. The liquid treating solution of claim 15 wherein the pH is from 2.0 to 4.8.
 17. The process of claim 1, in which a visually clean surface of an aluminum substrate is provided with a corrosion resistant surface by the sole step of contacting the aluminum substrate with a liquid treatment solution comprising water; (A) 0.1 to 20 mM/kg of a fluorine containing composition selected from the group consisting of tetrafluoroboric acid, water soluble, partially or totally neutralized salts of tetrafluoroboric acid, hexafluorotitanic acid, water soluble, partially or totally neutralized salts of hexafluorotitanic acid, hexafluorozirconic acid, water soluble, partial or totally neutralized salts of hexafluorozirconic acid, hexafluorohafinic acid, water soluble, partially or totally neutralized salts of hexafluorohafnic acid and mixtures thereof; the concentrations of any salts included in the solution being measured as their stoichiometric equivalent of the corresponding acid; and (B) from 0.40 to 95 mM/kg. of vanadate anions, measured as its stoichiometric equivalent as vanadium atoms.
 18. The process of claim 17 wherein the liquid treatment solution is at a pH of 1.0 to 5.5.
 19. The article of manufacture prepared by the process of claim
 1. 20. The article of manufacture of claim 19 wherein the surface of the article is dried directly after contact with the liquid solution to provide a coating containing from 0.1 to 8 mg/m² of vanadium. 